1. Field of the Invention
The present invention relates to a image stabilization control circuit for driving a vibration compensation mechanism provided in order to compensate for camera shake or other vibration in an image capturing device such as a digital still camera.
2. Description of the Prior Art(s)
Contemporary image capturing devices are often provided with camera shake correction functions in order to suppress a decline in picture quality due to camera shake. Many types of camera shake correction methods exist. In one of the methods, vibration in the image capturing device is detected by a vibration detector, and an optical component such as a correction lens, or an imaging element such as a CCD image sensor is displaced by an actuator on the basis of the detected signal. The vibration detector employs a gyro-sensor and detects angular velocity that corresponds to the change in the direction of the optical axis. The displacement magnitude of the lens or the like is used to controllably drive the actuator. Therefore, the image stabilization control circuit for generating the driving signal of the actuator performs a process in which the angular velocity or other type of displacement speed obtained from the vibration detector is integrated and converted to the displacement magnitude.
More particularly, the process for obtaining the displacement magnitude subjects the angular velocity signal outputted from the gyro-sensor to a camera shake component extraction process to remove a frequency component below the region of camera shake vibration frequencies, and converts the angular velocity into an angle-dependent displacement magnitude by integration. In the process for obtaining the displacement magnitude, by damping the output signal of the integration process or by other means, a centering process is also performed to establish the displacement magnitude so that it is made more difficult for the lens or the like to reach the movability limit. As used herein, the phrase “a processor for generating the vibration-compensating signal that corresponds to the displacement magnitude on the basis of the output signal of the gyro-sensor” is referred to as a gyro-equalizer.
Heretofore, gyro-equalizers have been implemented by software for which a microprocessor is used. In this case, a high processing rate is required for the image stabilization control circuit, and the microprocessor must be able to operate with a high speed clock. For instance, in the event that an imaging device is capturing 30 image frames per second to obtain moving images, it is necessary for the lens position to follow a vibration with a speed greater than 1/30th of a second.
Power consumption increases in the image stabilization control circuit in the event that a microprocessor is driven using a high speed clock. An image capturing device carrying a image stabilization control circuit is driven by a secondary battery such as a lithium-ion battery as a power source. Therefore, as the power consumption of the image stabilization control circuit is increased, the secondary battery depletes more rapidly, and the drive time of the image capturing device is reduced. In other words, a problem arises in which the time for capturing moving pictures is reduced, and the number of capturing still images decreases. Because the camera shake correction function in an image capturing device often operates not only when capturing moving pictures or still images but also during preview mode when an image is being prepared, consumption of power by the camera shake correction function should preferably be reduced.
In this case, by implementing a gyro-equalizer with a filter circuit, the microprocessor can be dispensed with and power consumption can be reduced. More specifically, a camera shake component extraction process can be configured using a high frequency pass filter (high pass filter, or HPF). It is possible to perform an integration process by using a low frequency pass filter (low pass filter, or LPF). It is also possible to perform a centering process by using an HPF and removing the direct-current component of the integration process output signal.
In the event that these filter circuits are used to compose the gyro-equalizer, it is desirable that the phase of the output signal of the gyro-equalizer be 90 degrees delayed from the input signal of the gyro-equalizer. In other words, the accuracy of the integration signal decreases as the phase delay is shifted from 90 degrees, which in turn causes the accuracy of the displacement magnitude to decline and the vibration to be less accurately compensated.
The output of the gyro-sensor is therefore delayed in phase in the high-frequency region. The problem has been that this phase delay causes the integration process performed by the gyro-equalizer to become less accurate. FIG. 4 shows typical phase characteristics illustrating this problem. FIG. 4 shows a phase characteristic (phase curve 70) of the gyro-equalizer as such, a phase characteristic (phase curve 72) of the output signal of the gyro-sensor, and a phase characteristic (phase curve 74) of the output signal of the gyro-equalizer in which the phase characteristic of the output signal of the gyro-sensor is reflected. The horizontal axis corresponds to the frequency f, and the vertical axis corresponds to the phase φ of the output signal in relation to the input signal. In FIG. 4, frequency fL is the lower limit of the target compensation region BCMP, and frequency fH is the upper limit. FIG. 4 shows that even if it is assumed that the phase characteristic (phase curve 70) of the gyro-equalizer as such is delayed by 90 degrees, the phase (phase curve 74) of the output signal thereof is affected by the phase delay (phase curve 72) of the output signal of the gyro-sensor, and is delayed even greater than 90 degrees.
In a vibration arising from camera shake or the like, the output signal of the gyro-equalizer may be comparatively weak in the high-frequency region, but the magnitude of the phase delay of the gyro-sensor increases as the frequency becomes higher. Therefore, there may be cases in which it is impossible to disregard the effect of the weak signal on the output of the gyro-equalizer. Particularly, this effect may be substantial in the case that the frequency region in which the gyro-sensor has a phase delay extends into the target compensation region BCMP in which the vibration component to be compensated is assumed to be present.